1. Field of the Invention
The present disclosure relates to a transmitting device and a transmitting method.
2. Description of the Related Art
In LTE (Long Term Evolution) Rel.8 (Release 8) developed by 3GPP (3rd Generation Partnership Project Radio Access Network), SC-FDMA (single-carrier frequency-division multiple-access) is adopted as an uplink communication system (for example, see 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation,” v.11.1.0, 3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding,” v.11.1.0, and 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures,” v.11.1.0). The SC-FDMA has a smaller PAPR (Peak-to-Average Power Ratio) and higher power usage efficiency at a user terminal, (UE: User Equipment).
In an uplink of the LTE, a data signal is transmitted in units of subframes using a PUSCH (Physical Uplink Shared Channel), and a control signal is transmitted in units of subframes using a PUCCH (Physical Uplink Control Channel) (for example, see 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation,” v.11.1.0). FIG. 1 illustrates a configuration example of a PUSCH subframe in a case that a normal cyclic prefix is used. As illustrated in FIG. 1, one subframe consists of two time slots. A plurality of SC-FDMA data symbols and one pilot symbol (referred to as a DMRS (Demodulation Reference Signal)) are time-multiplexed in each time slot. When receiving the PUSCH, the base station performs channel estimation using the DMRS. Then the base station demodulates and decodes the SC-FDMA data symbols using a channel estimation result. DFT-S-OFDM (Discrete-Fourier-Transform Spread Orthogonal Frequency Division Multiplexing) that is an extended version of the SC-FDMA can also be used in LTE-A (LTE-Advanced) Rel.10 (Release 10). In the DFT-S-OFDM, the PUSCH in FIG. 1 is divided into two spectra, and each spectrum is mapped to a different frequency bandwidth, thereby increasing a flexibility of scheduling.
The DMRS multiplexed on the PUSCH is generated based on a CAZAC (Constant Amplitude Zero Auto-Correlation) sequence having excellent auto-correlation characteristic and cross-correlation characteristic. 30 sequence groups are defined in the LTE. Each sequence group is generated by grouping the plurality of CAZAC sequences into one group (for example, see FIG. 2). Each sequence group includes the plurality of CAZAC sequences having a large correlation, and the plurality of CAZAC sequences have various sequence lengths. One of the 30 sequence groups is allocated to each cell based on a cell ID which is an ID specific to the cell. Therefore, different sequence groups having small correlation therebetween are allocated to different cells.
A user terminal generates the DMRS using the CAZAC sequence that has the sequence length corresponding to an allocated bandwidth among the plurality of CAZAC sequences. The plurality of CAZAC sequences are included in the sequence group which is allocated to the cell to which the user terminal belongs. And the user terminal time-multiplexes the DMRS on the PUSCH. Therefore, DMRSs having a large correlation are transmitted between the plurality of user terminals belonging to the same cell, and DMRSs having a small correlation are transmitted among a plurality of user terminals belonging to different cells. Because a correlation of the DMRSs is small between the cells, interference can be reduced by a window function method or by averaging, even if the interference is generated between the DMRSs transmitted at the same timing. On the other hand, in the same cell, the DMRSs of the plurality of user terminals are orthogonalized by allocating different frequency bandwidths or different time periods to the plurality of user terminals, which allows the interference not to be generated. The same frequency bandwidth or the same time period can be allocated to the plurality of user terminals (referred to as MU-MIMO (Multi-user multi-input multi-output)). In this case, the DMRSs of the user terminals can be orthogonally multiplexed by performing different cyclic shifts (CSs) to the DMRSs of the user terminals or multiplying two DMRSs in the PUSCH by different OCCs (Orthogonal Cover Codes) among the user terminals.
As described above, among a plurality of cells, an interference among the plurality of signals transmitted at the same time period can be reduced by using sequence groups different among the a plurality of cells, and spatial reuse of a wireless resource can be implemented. In one cell, the wireless resource can efficiently be used by applying the MU-MIMO. Therefore, high-efficiency uplink transmission can be implemented in the LTE.
Further, a virtual cell ID that is an ID different from the cell ID specific to the cell is added in LTE-A Rel.11 (Release 11). In the virtual cell ID, any sequence group can be allocated to any user terminal irrespective of the cell ID of the cell to which the user terminal belongs.
However, conventionally, in the case that the DMRSs are transmitted among the plurality of cells using different mapping patterns, sometimes the DMRSs can cause an interference to a DMRS of another user terminal belonging to another cell such as a peripheral cell including a neighbor cell.